Double-wall pipe and refrigerant cycle device using the same

ABSTRACT

A double-wall pipe includes an outer pipe provided with first and second openings, respectively, at first and second end parts of the outer pipe in a pipe longitudinal direction, and an inner pipe inserted in the outer pipe to define a passage between the outer pipe and the inner pipe. An inlet portion is connected to the outer pipe to communicate with the passage through the first opening, and an outlet portion is connected to the outer pipe to communicate with the passage through the second opening. In the double-wall pipe, the outer pipe and the inner pipe can be disposed to define an expanded portion having an expanded sectional area in the first passage, and the expanded portion can be provided at least at a portion near the inlet portion and the outer portion. The inner pipe can be provided with plural grooves in the double-wall pipe.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Applications No.2004-325522 filed on Nov. 9, 2004, No. 2004-325521 filed on Nov. 9,2004, No. 2005-112825 filed on Apr. 8, 2005, No. 2005-136390 filed onMay 9, 2005, and No. 2005-263967 filed on Sep. 12, 2005, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a double-wall pipe constructed with aninner pipe defining an inner passage and an outer pipe enveloping theinner pipe so as to define an outer passage between the outer pipe andthe inner pipe. The double-wall pipe can be suitably used for arefrigerant cycle device, for example.

BACKGROUND OF THE INVENTION

A double-wall pipe is used in a refrigerant cycle device for a vehicleair conditioning system, for example.

A double-wall pipe disclosed in, for example, JP-A-2001-277842 is formedby combining a high-pressure refrigerant pipe extending between acompressor and a condenser and between the condenser and an evaporator,and a low-pressure refrigerant pipe extending between the evaporator andthe compressor. The double-wall pipe has at least a double-wall sectionformed by enveloping the low-pressure refrigerant pipe (or thehigh-pressure refrigerant pipe) by the high-pressure refrigerant pipe(or the low-pressure refrigerant pipe).

Heat of the high-temperature high-pressure refrigerant can betransferred to the low-temperature low-pressure refrigerant in thedouble-wall section. Thus, the high-pressure refrigerant is super-cooled(subcooled) by the low-pressure refrigerant and, consequently, therefrigerant having an increased liquid refrigerant amount flows into theevaporator. Resistance of the evaporator against the flow of therefrigerant decreases with the increase of the liquid refrigerant amountof the refrigerant. Consequently, the cooling ability of a coolingsystem including the evaporator is enhanced. The low-pressurerefrigerant discharged from the evaporator is superheated by the heat ofthe high-pressure refrigerant to prevent the liquid compression in thecompressor.

A double-wall pipe disclosed in JP-A-2003-329376 is formed by combiningan inner pipe of a first diameter and an outer pipe of a seconddiameter. This double-wall pipe is fabricated by inserting the innerpipe into the outer pipe, and twisting the inner pipe so that a screwthread formed by twisting the inner pipe is pressed against the insidesurface of the outer pipe.

A first fluid flows through the inner pipe and a second fluid flowsthrough a helical passage defined by the screw thread of the inner pipeand the outer pipe.

The double-wall pipe disclosed in JP-A-2001-277842 enables heat transferfrom the high-pressure refrigerant to the low-pressure refrigerant.However, nothing about heat transfer efficiency is described inJP-A-2001-277842. Heat transfer efficiency can be increased byincreasing the outside diameter of the inner pipe close to the insidediameter of the outer pipe to increase the area of the heat transfersurface.

However, when the outside diameter of the inner pipe is close to theinside diameter of the outer pipe, an annular passage formed between theinner and the outer pipe is very narrow and exerts high resistanceagainst the flow of the refrigerant. Moreover, an inlet and an outletformed at opposite end parts of the outer pipe, or an inlet or an outletformed at one end part of the outer pipe increases resistance againstthe flow of the refrigerant flowing in the vicinity of the inlet and theoutlet or in the vicinity of the inlet or the outlet.

Since the inner pipe has a small surface area, heat cannot beefficiently transferred from one to the other of the fluids respectivelyflowing through the inner pipe and through the passage between the innerand the outer pipe.

The double-wall pipe disclosed in JP-A-2003-329376 connects headersrespectively to an inlet and an outlet opening into the helical passageand formed respectively in opposite end parts of the outer pipe. Thus,the double-wall pipe needs additional parts.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an improveddouble-wall pipe.

A second object of the present invention is to provide a double-wallpipe having an outer pipe and an inner pipe connected to the outer pipeto form a passage between the outer pipe and the inner pipe, andfacilitating connection of pipes to the passage.

A third object of the present invention is to provide a double-wall pipehaving an outer pipe and an inner pipe connected to the outer pipe toform a passage between the outer pipe and the inner pipe, and providedwith joints with a low resistance on a fluid flowing therethrough.

A fourth object of the present invention is to provide a double-wallpipe capable of efficiently transferring heat from one fluid to another.

A fifth object of the present invention is to provide a double-wall pipecapable of efficiently transferring heat between a high-temperaturehigh-pressure refrigerant and a low-temperature low-pressure refrigerantin a refrigerant cycle device.

A sixth object of the present invention is to provide a refrigerantcycle device having a double-wall pipe.

According to an aspect of the present invention, a double-wall pipeincludes an outer pipe provided with first and second openings,respectively, at first and second end parts of the outer pipe in a pipelongitudinal direction, an inner pipe inserted in the outer pipe todefine a passage between the outer pipe and the inner pipe, an inletportion connected to the outer pipe to communicate with the passagethrough the first opening, and an outlet portion connected to the outerpipe to communicate with the passage through the second opening.

In the double-wall pipe, the outer pipe and the inner pipe are disposedto define an expanded portion having an expanded sectional area in thepassage between the outer pipe and the inner pipe, and the expandedportion is provided at least at a portion near the inlet portion and theouter portion. Accordingly, the double-wall pipe can be formed into asimple structure, and the expanded portion reduces resistance againstthe flow of a fluid flowing in the vicinity of the inlet portion and theoutlet portion. Consequently, the fluid is able to flow at high flowrates through the passage between the inner pipe and the outer pipe, andheat can be efficiently transferred between a fluid flowing in the innerpipe and a fluid flowing through the passage between the inner pipe andthe outer pipe.

The expanded portion can be provided by expanding at least a part of acircumferential portion of the outer pipe in a circumferentialdirection, or can be provided by reducing at least a part of acircumferential portion of the inner pipe in a circumferentialdirection, at least at a portion near the inlet portion and the outletportion.

According to another aspect of the present invention, in a double-wallpipe constructed with an outer pipe and an inner pipe inserted into theouter pipe, a surface of the inner pipe is provided with a plurality ofgrooves. For example, the grooves are straight grooves extending in alongitudinal direction of the inner pipe, or helical grooves windingaround the inner pipe and extending in the longitudinal direction of theinner pipe. Alternatively, the grooves can include straight groovesextending in the longitudinal direction of the inner pipe, and helicalgrooves winding around the inner pipe and extending in the longitudinaldirection of the inner pipe. Furthermore, the helical grooves caninclude first helical grooves winding in a first direction around theinner pipe, and second helical grooves winding in a second directionopposite the first direction around the inner pipe.

Accordingly, a turbulent flow of the fluid can be easily generated inthe passage between the inner pipe and the outer pipe, and the turbulentflow of the fluid enhances the heat transfer efficiency. Consequently,heat can be efficiently transferred between the fluid flowing throughthe inner pipe and the fluid flowing through the passage between theinner pipe and the outer pipe.

According to another aspect of the present invention, in a double-wallpipe constructed with an outer pipe and an inner pipe inserted into theouter pipe, the inner pipe is provided in its wall with a groove portionextending from a first end part to a second end part of the inner pipe,the outer pipe has a first joining part joined airtightly to the innerpipe at the first end part, and the outer pipe has a first connectinghole which is opened in a radial direction to directly communicate withthe groove portion at the first end part. In this case, the double-wallpipe can be easily formed with a simple structure. The outer pipe can beprovided with a second joining part joined airtightly to the inner pipeat the second end part, and a second connecting hole can be opened inthe outer pipe in the radial direction to directly communicate with thegroove at the second end part.

For example, the groove portion has a groove extending in acircumferential direction at least in a part corresponding to theconnecting hole of the outer pipe. In this case, the groove can extendin a complete circle in the circumferential direction at least in thepart corresponding to the connecting hole of the outer pipe.Furthermore, the groove portion can include a helical groove extendinghelically, or/and a straight groove extending from the first end part tothe second end part.

The inner pipe can be provided with cylindrical end parts respectivelyformed in first and second end parts thereof, the outer pipe can beprovided with cylindrical end parts formed at first and second end partsthereof. In this case, the outer pipe has an inside diameter slightlygreater than an outside diameter of the cylindrical end parts of theinner pipe, and the cylindrical end parts of the outer pipe are directlyairtightly joined to the cylindrical end parts of the inner pipe,respectively, to form joints. Furthermore, the outer pipe including thecylindrical end parts can have a fixed inside diameter. Alternatively,parts, forming the joints, of the cylindrical end parts of the outerpipe can be radially reduced so as to tightly contact the inner pipe.

The double-wall pipe can be used in a refrigerant cycle device includinga compressor, a condenser, a pressure-reducing device and an evaporator.In this case, the passage between the outer pipe and the inner pipe, anda passage inside the inner pipe can be used, respectively, as at least apart of a high-pressure passage connecting the condenser and thepressure-reducing device to carry a high-pressure refrigerant, and as atleast a part of a low-pressure passage connecting the evaporator and thecompressor to carry a low-pressure refrigerant. That is, the outer pipeand the inner pipe of the double-wall pipe can be used for a refrigerantcycle device such that a high-pressure refrigerant before beingdecompressed in the pressure-reducing unit flows through the passagebetween the outer pipe and the inner pipe, and a low-pressurerefrigerant after being decompressed in the pressure-reducing unit flowsin the inner pipe.

In a double-wall pipe, an uneven portion including at least a groove canbe provided in the inner pipe. For example, the uneven portion includesridges and grooves relative to an outer surface of the inner pipe, edgesof the ridges of the uneven portion of the inner pipe are rounded in aradius smaller than an inner radius of the outer pipe, and a passage canbe defined by the outer pipe and the grooves of the inner pipe and bythe outer pipe and the ridges of the inner pipe. The inner pipe can beprovided with an inner cylindrical end part without having the unevenportion, at one end part of the inner pipe, and the outer pipe can beprovided with an outer cylindrical end part at a part corresponding tothe inner cylindrical end part of the inner pipe. In this case, aninside diameter of the outer pipe can be set slightly greater than anoutside diameter of the inner cylindrical end part of the inner pipe,and the outer cylindrical end part of the outer pipe and the innercylindrical end part of the inner pipe can be directly airtightly joinedto form a joint. Therefore, the joint can be easily formed.

In the present invention, the double-wall pipe can be suitably used fora refrigerant cycle device, and the refrigerant cycle device having adouble-wall pipe can be suitably used for an air conditioner for avehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments made with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of an automotive air conditioning system;

FIG. 2 is a schematic perspective view of a refrigerant cycle devicemounted on a vehicle;

FIG. 3 is a partly sectional plan view of a double-wall pipe in a firstembodiment according to the present invention;

FIG. 4 is a sectional view taken on the line IV-IV in FIG. 3;

FIG. 5 is a Mollier diagram for explaining a phenomenon that occurs in adouble-wall pipe;

FIG. 6 is a partly sectional plan view of a double-wall pipe in a secondembodiment according to the present invention;

FIG. 7 is a partly cutaway perspective view of a double-wall pipe in athird embodiment according to the present invention;

FIG. 8 is a partly cutaway perspective view of a double-wall pipe in afourth embodiment according to the present invention;

FIG. 9 is a side view of an inner pipe included in a double-wall pipe ina fifth embodiment according to the present invention;

FIG. 10 is a sectional view taken on the line X-X in FIG. 9;

FIG. 11 is a partly sectional plan view of a double-wall pipe in a sixthembodiment according to the present invention; and

FIG. 12 is a sectional view showing a double-wall pipe according to amodification of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

In this embodiment, a double-wall pipe 160 for carrying a refrigerant istypically used for a refrigerant cycle device 100A for a vehicle airconditioning system 100. The double-wall pipe 160 will be described withreference to FIGS. 1 to 4.

A vehicle has an engine room 1 holding an engine 10 therein and apassenger compartment 2 separated from the engine room 1 by a dash panel3. The air conditioning system 100 has the refrigerant cycle device 100Aincluding an expansion valve 130 and an evaporator 140, and an interiorunit 100B. Components of the refrigerant cycle device 100A excluding theexpansion valve 130 and the evaporator 140 are disposed in apredetermined mounting space of the engine room 1. The interior unit100B is arranged in an instrument panel placed in the passengercompartment 2.

The interior unit 100B has components including a blower 102, theevaporator 140 and a heater 103, and an air conditioner case 101 housingthe components of the interior unit 100B. The blower 102 takes inoutside air or inside air selectively and sends air to the evaporator140 and the heater 103. The evaporator 140 is a cooling heat exchangerthat evaporates a refrigerant, that is, a fluid, used for arefrigeration cycle to make the evaporating refrigerant absorb latentheat of vaporization from air so as to cool the air. The heater 103 useshot water (engine-cooling water) for cooling the engine 10 as a heatsource to heat air to be blown into the passenger compartment 2.

An air mixing door 104 is disposed near the heater 103 in the airconditioner case 101. The air mixing door 104 is operated to adjust themixing ratio between cool air cooled by the evaporator 140 and hot airheated by the heater 103 so that air having a desired temperature issent into the passenger compartment 2.

The refrigerant cycle device 100A includes a compressor 110, a condenser120, the expansion valve 130 and the evaporator 140. Pipes 150 connectthose components of the refrigerant cycle device 100A to form a closedcircuit. A double-wall pipe 160 of the present invention is placed inthe pipes 150, for example.

The compressor 110 is driven by the engine 10 to compress a low-pressurerefrigerant vapor to provide a high-temperature high-pressurerefrigerant vapor. A pulley 111 is attached to the drive shaft of thecompressor 110. A drive belt 12 is extended between the pulley 111 and acrankshaft pulley 11 to drive the compressor 110 by the engine 10. Thepulley 111 is linked to the drive shaft of the compressor 110 by anelectromagnetic clutch (not shown). The electromagnetic clutch connectsthe pulley 111 to or disconnects the pulley 111 from the drive shaft ofthe compressor 110. The condenser 120 is connected to a discharge sideof the compressor 110. The condenser 120 is a heat exchanger(refrigerant radiator) that cools the refrigerant vapor by fresh air(outside air) to condense the refrigerant vapor into liquid refrigerant.

The expansion valve 130 reduces the pressure of the refrigerant (e.g.,liquid refrigerant) discharged from the condenser 120 and makes therefrigerant expand. The expansion valve 130 is a pressure-reducing valvecapable of reducing the pressure of the refrigerant in an isoentropicstate. The expansion valve 130 is placed near the evaporator 140 in thepassenger compartment 2. The expansion valve 130 is atemperature-controlled expansion valve having a variable orifice and iscapable of controlling the flow of the refrigerant so that therefrigerant is heated at a predetermined degree of superheat. Theexpansion valve 130 controls the expansion of the refrigerant so thatthe degree of superheat of the refrigerant in the evaporator 140 is, forexample, 5° C. or below, more specifically, in the range of 0° C. to 3°C. As described above, the evaporator 140 is a cooling heat exchangerfor cooling air to be blown into the passenger compartment. Thedischarge side of the evaporator 140 is connected to the suction side ofthe compressor 110.

The double-wall pipe 160 is placed in the pipe 150 extending between thecondenser 120 and the expansion valve 130. The double-wall pipe 160constructs a part of a high-pressure pipe 151 for carrying thehigh-pressure refrigerant discharged from the compressor 110, and a partof a low-pressure pipe 152 for carrying the low-pressure refrigerantfrom the evaporator 140 to the compressor 110.

The double-wall pipe 160 has a length in the range of 700 to 900 mm. Asshown in FIG. 2, the double-wall pipe 160 has a plurality of bendportions 160 c and is extended in the engine room 1 so that thedouble-wall pipe 160 may not touch the engine 10 and other devices andthe body of the vehicle.

Referring to FIGS. 3 and 4, the double-wall pipe 160 has an outer pipe161 and an inner pipe 162. The inner pipe 162 is extended in the outerpipe 161 to penetrate through the outer pipe 161. The outer pipe 161 is,for example, a 6/8 inch aluminum pipe having an outside diameter of19.05 mm and an inside diameter of 16.65 mm. Longitudinal end parts ofthe outer pipe 161 are reduced to form reduced joining parts 161 b. Thereduced joining parts 161 b of the outer pipe 161 are welded to theinner pipe 162 in a liquid-tight or air-tight state. Thus, the outerpipe 161 and the inner pipe 162 define a passage 160 a therebetween.

An inlet pipe 163 (i.e., an inlet portion) and an outlet pipe 164 (i.e.,an outlet portion) are welded to end parts of the outer pipe 161,respectively. The refrigerant flows through the inlet pipe 164 into thepassage 160 a and flows out of the passage 160 a through the outlet pipe164. The inlet pipe 163 and the outlet pipe 164 extend perpendicularlyto the longitudinal direction of the outer pipe 161 and serve asconnecting pipes. Joints 163 a and 164 a are attached to the inlet pipe163 and the outlet pipe 164, respectively. The joint 163 a connects theinlet pipe 163 and the high-pressure pipe 151 connected to the condenser110. The joint 164 a connects the outlet pipe 164 to the high-pressurepipe 151 connected to the expansion valve 130. Therefore, thehigh-pressure refrigerant flows through the passage 160 a.

Parts corresponding to the inlet pipe 163 and the outlet pipe 164 of theouter pipe 161 are expanded to form expanded parts 161 a. The expandedparts 161 a form expanded passages 160 b having an increased sectionalarea in the passage 160 a.

The inner pipe 162 is, for example, a ⅝ in. aluminum pipe having anoutside diameter of 15.88 mm and an inside diameter of 13.48 mm. Theoutside diameter of the inner pipe 162 is determined, so that thepassage 160 a has a sectional area large enough to pass thehigh-pressure refrigerant, and the outer surface of the inner pipe 162is as close to the inner surface of the outer pipe 161 as possible.Thus, the heat transfer surface area of the inner pipe 162 can beeffectively increased.

Joints 162 c are attached to the opposite longitudinal ends of the innerpipe 162, respectively. The low-pressure pipe 152 connected to theevaporator 140 is connected to the joint 162 c on the right side, asviewed in FIG. 3, and the low-pressure pipe 152 connected to thecompressor 110 is connected to the joint 162 c on the left side, asviewed in FIG. 3. The low-pressure refrigerant flows through the innerpipe 162 as in the arrow in FIG. 3.

Three longitudinal straight grooves 162 b are formed on the surface of apart, corresponding to the range where the passage 160 a is formed, ofthe inner pipe 162 as shown in FIG. 4. Thus, the straight grooves 162 band longitudinal straight ridges protruding outside are arrangedalternately in the circumferential direction. The straight grooves 162 bform longitudinal straight inner ridges protruding inside the inner pipe162. The straight grooves 162 b and the straight ridges each of whichextends in the pipe longitudinal direction are arranged alternatively inthe circumferential direction. In FIG. 4, three straight grooves 162 band straight ridges are provided as an example.

The operation and functional effect of the double-wall pipe 160 will bedescribed in connection with a Mollier diagram shown in FIG. 5.

When the passenger requests to operate the air conditioning system 100for a cooling operation, the electromagnetic clutch is engaged to drivethe compressor 110 by the engine 10. Then, the compressor 110 draws therefrigerant discharged from the evaporator 140, compresses the drawnrefrigerant and discharges high-temperature high-pressure refrigeranttoward the condenser 120. The condenser 120 cools the high-temperaturehigh-pressure refrigerant into the liquid refrigerant in a substantiallytotally liquid phase, for example. The liquid refrigerant flows throughthe passage 160 a into the expansion valve 130. The expansion valve 130reduces the pressure of the liquid refrigerant and expands the liquidrefrigerant. The evaporator 140 evaporates the liquid refrigerant into agaseous refrigerant of a substantially saturated gas having a degree ofsuperheat in the range of 0° C. to 3° C. The refrigerant evaporated bythe evaporator 140 absorbs heat from air flowing through the evaporator140 so that the air is cooled. The saturated gaseous refrigerantevaporated by the evaporator 140, having a low-temperature low-pressure,flows through the inner pipe 162, and returns to the compressor 110.

Heat is transferred from the high-temperature high-pressure refrigerantflowing through the passage 160 a to the low-temperature low-pressurerefrigerant flowing through the inner pipe 162 by performing heatexchange therebetween. Consequently, the high-temperature high-pressurerefrigerant is cooled and the low-temperature low-pressure refrigerantis superheated as shown in FIG. 5. Thus, the liquid-phase refrigerantdischarged from the condenser 120 is super-cooled (sub-cooled) and thetemperature thereof drops while the high-pressure refrigerant from thecondenser 120 is flowing through the double-wall pipe 160 (sub-cooling).The saturated gaseous refrigerant (low-pressure refrigerant) dischargedfrom the evaporator 140 is superheated to a gaseous refrigerant having adegree of superheat (superheating).

The parts of the outer pipe 161 of the double-wall pipe 160 in thisembodiment are expanded to form the expanded parts 160 b. Therefore, theinlet pipe 163 and the outlet pipe 164 can be simply connected to theouter pipe 161 so as to communicate with the passage 160 a.

Since the outside diameter of the inner pipe (e.g., ⅝ in. pipe) 162 isdetermined so that the passage 160 a has a sectional area large enoughto pass the high-pressure liquid-phase refrigerant, and the outersurface of the inner pipe 162 is as close to the inner surface of theouter pipe (e.g., 6/8 in. pipe) 161 as possible, the inner pipe 162 hasa large heat transfer surface area. Consequently, heat can beefficiently transferred from the high-temperature refrigerant to thelow-temperature refrigerant.

The expanded parts 161 a formed in the outer pipe 161 form the expandedpassages 160 a, respectively, and the inlet pipe 163 and the outlet pipe164 are connected to the expanded parts 161 a, respectively. Therefore,impact exerted on the inner pipe 162 by the high-pressure refrigerantflowing from the inlet pipe 163 into the passage 160 a, resistanceagainst the circumferential flow of the refrigerant flowing around theinner pipe 162, and resistance against the flow of the refrigerantdeflecting from a longitudinal direction to a circumferential directionand flowing into the outlet pipe 164 can be reduced. Consequently, thehigh-pressure refrigerant can flow at a high flow rate through thepassage 160 a, and the heat can be efficiently transferred from thehigh-temperature refrigerant (i.e., high-pressure refrigerant) to thelow-temperature refrigerant (i.e., low-pressure refrigerant).

Further, in this embodiment, the high-temperature high-pressurerefrigerant flows through the passage 160 a between the outer pipe 161and the inner pipe 162. Therefore, it can restrict a heat loss due to aheat exchange between high-temperature air in the engine room 1 and thelow-temperature low-pressure refrigerant inside the inner pipe 162.Accordingly, heat transmitting performance between the high-pressurehigh-temperature refrigerant and the low-temperature low-pressurerefrigerant can be effectively improved. As a result, it is unnecessaryto provide an insulating material on the outer surface of the outer pipe161, for insulating a heat exchange between the low-temperaturelow-pressure refrigerant and high-temperature air in the engine room 1.

The hardness of the inner pipe 162 can be increased by work hardeningwhen the straight grooves 162 b are formed, and the bending rigidity(the section modulus) of the inner pipe 162 can be increased by thelongitudinal ribs formed when the straight grooves 162 b are formed.Consequently, the sectional deformation of the inner pipe 162 when thebend portion 160 c is formed in the double-wall pipe 160, and theresulting narrowing of the passage 160 can be suppressed. Since thestraight grooves 162 b increase the sectional area of the passage 160 a,the flow resistance of the high-pressure refrigerant can be reduced.Therefore, the flow rate of the high-pressure refrigerant flowingthrough the passage 160 a can be increased and the efficiency of heattransfer from the high-temperature refrigerant (i.e., high-pressurerefrigerant) to the low-temperature refrigerant (i.e., low-pressurerefrigerant) can be improved.

The straight grooves 162 b increase the area of the surface of the innerpipe 162 serving as a heat transfer surface for transferring heat fromthe high-temperature high-pressure refrigerant flowing through thepassage 160 a to the low-temperature low-pressure refrigerant flowingthrough the inner pipe 162. Consequently, the efficiency of heattransfer from the high-temperature refrigerant to the low-temperaturerefrigerant can be improved. The straight grooves 162 b form thelongitudinal straight inner ridges inside the inner pipe 162, and thestraight grooves 162 b and the protruding portions are arrangedcircumferentially alternately on the outer surface of the inner pipe162. Therefore, heat can be satisfactorily transferred from thehigh-temperature refrigerant flowing through the passage 160 a throughthe inner pipe 162 to the low-temperature refrigerant flowing throughthe inner pipe 162.

The high-temperature high-pressure refrigerant flows through the passage160 a and the low-temperature low-pressure refrigerant flows through theinner pipe 162. Therefore, heat loss between high-temperature air in theengine room 1 and the low-pressure refrigerant can be prevented, and theheat can be efficiently transferred from the high-temperaturerefrigerant to the low-temperature refrigerant.

When the outer pipe 161 and the inner pipe 162 of the double-wall pipe160 are formed integrally by an extrusion process, plural longitudinalextending ribs are formed between the outer pipe 161 and the inner pipe162 in a circumferential arrangement, and the longitudinal extendingribs divide the passage 160 a into a plurality of divisional passages.In this case, the longitudinal extending ribs exert resistance againstthe flow of the refrigerant in the passage 160 a. When a part, facingone of the divisional passages, of the outer pipe 161 is brought intocontact with a part, facing the divisional passage, of the inner pipe162 when the bend portion 160 c is formed in the double-wall pipe 160,the divisional passage is closed and, consequently, resistance againstthe flow of the refrigerant increases. In the first embodiment, theouter pipe 161 and the inner pipe 162 are produced separately and arecombined to form the double-wall pipe 160, the foregoing problem doesnot arise in the double-wall pipe 160.

Normally, the temperature difference between air and the refrigerant issmall and the heat exchanging performance (cooling ability) reduces whenthe refrigerant flowing into the evaporator 140 has a superheatingdegree. The double-wall pipe 160 in this embodiment can give a degree ofsuper heat to the refrigerant discharged from the evaporator 140 andhence it is unnecessary to have a degree of super heat to therefrigerant (saturated gas) flowing into the evaporator 140. Therefore,the evaporator 140 is able to exercise a high heat exchangingperformance (cooling ability), and the double-wall pipe 160 gives adegree of super heat to the refrigerant discharged from the evaporator140 to convert the refrigerant into a perfectly gaseous refrigerant(gas-phase refrigerant). Consequently, it is possible to prevent thecompression of the liquid-phase refrigerant by the compressor 110.

The expanded parts 161 a may be formed in circumferential parts, nearthe inlet pipe 163 and the outlet pipe 164, of the outer pipe 161,depending on resistance against the flow of the high-pressurerefrigerant in the vicinity of the inlet pipe 163 and the outlet pipe164.

Second Embodiment

FIG. 6 shows a part of a double-wall pipe 160 of the second embodiment.

Referring to FIG. 6, the double-wall pipe 160 in the second embodimentaccording to the present invention has expanded passages 160 b formed inlongitudinal end parts thereof and different from those of thedouble-wall pipe 160 in the first embodiment.

Since the expanded passages 160 b formed near an inlet pipe 163 and anoutlet pipe 164 in the longitudinal end parts of the double-wall pipe160, respectively, are the same in shape, only the expanded passage 160b formed near an inlet pipe 163 will be described. A depression 162 a(recess portion) is formed in an inner pipe 162 by radially depressing acircumferential part of the inner pipe 162 to define the expandedpassage 160 b. Because the depression 162 a is formed in the inner pipe162, a narrow part is formed in the inner pipe 162 due to the depression162 a. The expanded passages 160 b at the junction between the inletpipe 163 and a passage 160 a defined by the outer pipe 161 and the innerpart 162 and at the junction between the outlet pipe 164 and the passage160 a can be formed by forming the depressions 162 a in the inner pipe162 without diametrically expanding the end parts of the outer pipe 161.The depressions 162 a are formed in the circumferential parts of theinner part 162 in a circumferential range. The depressions 162 a may beannular grooves formed in the end parts of the inner pipe 162. Thedepression 162 a at the junction of the inlet pipe 163 and the passage160 a guides the refrigerant having passed through the inlet pipe 163into grooves 162 b. The depression 162 a at the junction of the outletpipe 164 and the passage 160 a guides the refrigerant having passedthrough the passage 160 a into outlet pipe 164. Thus, in the secondembodiment, the effect of the double-wall pipe 160 similarly to that ofthe double-wall pipe 160 in the first embodiment can be obtained.

The depressions 162 a may be annular grooves in parts, near the inletpipe 163 and the outlet pipe 164, of the inner pipe 162, depending onresistance against the flow of the high-pressure refrigerant in thevicinity of the inlet pipe 163 and the outlet pipe 164.

In the second embodiment, the other parts can be made similar to thoseof the above-described first embodiment.

Third Embodiment

FIG. 7 shows an inner pipe 160 and an outer pipe 161 of the thirdembodiment. Referring to FIG. 7, a double-wall pipe 160 in the thirdembodiment according to the present invention has an inner pipe 162provided with three helical grooves 162 d formed in the shape of athree-thread screw instead of the straight grooves 162 a of the innerpipe 162 of the double-wall pipe 160 in the first embodiment. Multiplehelical grooves, that is, more than one helical groove, may be formed inthe shape of a multithread screw and arranged at equal or predeterminedpitches or a single helical groove may be formed in the inner pipe 162instead of the three helical grooves 162 d. The three helical grooves162 d are formed by deforming the wall of the inner pipe 162. The threehelical grooves 162 d form helical ridges inside the inner pipe 162. Thethree helical grooves 162 d are parallel to each other.

The three helical grooves 162 d winding around the inner pipe 162increase the bending rigidity (the section modulus) of the inner pipe162 and prevent an undesirable deformation in the section of the innerpipe 162 when a bend portion 160 c (FIG. 2) is formed in the double-wallpipe 160.

Turbulence can be caused in the refrigerant flowing through a passage160 a due to the spiral grooves 162 d, thereby enhancing heat transferefficiency. Consequently, heat can be efficiently transferred between afluid (e.g., low-pressure refrigerant) inside the inner pipe 162 and afluid (e.g., high-pressure refrigerant) in the passage 160 a.

In the third embodiment, the other parts can be made similar to those ofthe above-described first or second embodiment.

Fourth Embodiment

Referring to FIG. 8, an inner pipe 162 included in a double-wall pipe160 in the fourth embodiment is provided with straight grooves 162 b andhelical grooves 162 d. That is, the fourth embodiment is a combinationbetween the third embodiment and the first embodiment, in the structureof the inner pipe 162. In the fourth embodiment, the other parts can beformed similar to the first embodiment or the second embodiment.

Fifth Embodiment

The inner pipe 162 of the double-wall pipe 160 in the third embodimentis provided with the helical grooves 162 d parallel to each other. Theinner pipe 162 may be provided with helical grooves respectively havingdifferent helix angles and intersecting each other. When the inner pipe162 is provided with such helical grooves intersecting each other,turbulent streams of the refrigerant can be produced in the passage 160a and in the inner pipe 162 to promote heat transfer. The inner pipe 162may be provided with a plurality of helical grooves respectively havingpositive and negative helical angles. For example, one of two helicalgrooves may be a right-hand helical groove and the other may be aleft-hand helical groove, or some of a plurality of helical grooves maybe right-hand helical grooves and the rest may be left-hand helicalgrooves. The inner pipe 162 may be provided with a plurality of parallelright-hand helical grooves and a plurality of parallel left-hand helicalgrooves. FIGS. 9 and 10 show an inner pipe 162 included in a double-wallpipe 160 in the fifth embodiment according to the present invention in aside elevation and a cross-sectional view, respectively. In FIG. 9,broken lines indicate center lines of two first helical grooves 162 e,that is, right-hand helical grooves 162 e, and two second helicalgrooves 162 f, that is, left-hand helical grooves 162 f, formed in theinner pipe 162. The numbers, widths, depths, helix angles and pitches ofthe first helical grooves 162 e and the second helical grooves 162 f maybe determined on the basis of the sectional area of the passage 160 a,the resistance of the passage 160 a on the flow of the refrigerant andthe flexibility of the inner pipe 162. The inner pipe 162 may beprovided with straight grooves in combination with the helical grooves162 e and 162 f.

When the right-hand helical grooves 162 e and the left-hand helicalgrooves 162 f are formed by deforming the wall of the inner pipe 162,the inner pipe 162 assumes the shape of a bellows and the inner pipe 162can be easily bent in any directions. The grooves formed in the innerpipe 162 form a plurality of ridges and recesses inside the inner pipe162. Consequently, heat transfer between a fluid (refrigerant) insidethe inner pipe 162 a fluid (refrigerant) flowing through outside theinner pipe 162 can be promoted. The inner pipe 162 has the plurality ofgrooves and a plurality of protrusions in an alternate arrangement.Consequently, heat transfer between the fluid flowing through thepassage 160 a and the fluid inside the inner pipe 162 can be promoted.In the double-wall pipe 160 in the fifth embodiment, the helical grooves162 e and 162 f formed in the inner pipe 162 form a plurality ofjunctions and a plurality of rhombic protruding portions on the surfaceof the inner pipe 162. The rhombic protruding portions are in contactwith the inside surface of an outer pipe 161. Thus, the passage 160 canbe surely formed between the outer pipe 161 and the inner pipe 162. Asshown in FIG. 10, the edges of the ridges each formed between thehelical grooves 162 e and 162 f are rounded in a radius of a circlesmaller than the radius of a circle enveloping the inner pipe 162 in astate before the helical grooves 162 e and 162 f are formed. Thus, thearea of contact between the outer pipe 161 and the inner pipe 162 can bemade small.

Sixth Embodiment

FIG. 11 shows a double-wall pipe 160 in the sixth embodiment accordingto the present invention. This double-wall pipe 160 can be intended forcarrying a refrigerant in a refrigerant cycle device for an automotiveair conditioning system. The double-wall pipe 160 can be used as aninternal heat exchanger for transferring heat from a high-temperaturehigh-pressure refrigerant to a low-temperature low-pressure refrigerant.The double-wall pipe 160 in the sixth embodiment differs from thedouble-wall pipe 160 in the first embodiment principally in an outerpipe 161 of a shape different from that of the outer pipe 161 of thefirst embodiment and an inner pipe 162 provided with grooves of a shapedifferent from that of the grooves 162 b of the inner pipe 162 of thesecond embodiment.

The outer pipe 161 has a fixed inside diameter slightly greater than theoutside diameter of the inner pipe 162. End parts of the outer pipe 161are airtightly joined to end parts of the inner pipe 162 by airtightjoints 161 b. Each of the airtight joints 161 b is formed by connectinga cylindrical end part 161 c of the outer pipe 161 and a cylindrical endpart 162 h of the inner pipe 162. The cylindrical end parts 161 c of theouter pipe 161 are put on and joined by blazing or welding to thecylindrical end parts 162 h of the inner pipe 162, respectively, to formthe airtight joints 161 b. The radial dimensions of the cylindrical endparts 161 c of the outer pipe 161 are reduced by pressing so that thecylindrical end parts 162 h of the inner pipe 162 can be closely fittedto the cylindrical end parts 161 c, respectively.

The airtight joint 161 b may be formed in one end part of the outer pipe161 and one end part of the inner pipe 162, and the other ends of theouter pipe 161 and the inner pipe 162 may be connected by a joiningmeans other than the airtight joint 161 b. For example, a rubber O-ringmay be squeezed between the other end parts of the outer pipe 161 andthe inner pipe 162, or the other ends of the outer pipe 161 and theinner pipe 162 may be connected by a pipe joint.

Burring holes to be used as connecting holes are formed in the end partsof the outer pipe 161 at positions at a predetermined distance from theends of the outer pipe 161. The burring holes are provided to correspondto radial side portions of ends of a helical groove 162 d formed in theinner pipe 162 or annular grooves 162 g formed in the end parts of theinner pipe 162, respectively. Burrs extend radially outward from theedges of the burring holes, respectively. A flange inlet pipe 163 b anda flange outlet pipe 164 b are joined to the burring holes,respectively. The flange inlet pipe 163 b and the flange outlet pipe 164b open into the interior of the outer pipe 161. In this embodiment, theburring holes and the pipes 163 b, 164 b construct communication partscommunicating with components in the refrigerant cycle.

The inner pipe 162 has a fixed inside diameter. The inner pipe 162 hascylindrical end parts of a predetermined length. The inner pipe 162 is awave pipe (corrugated pipe) having outer ridges, outer grooves, innerridges and inner grooves. The ridges and the grooves are formedcircumferentially alternately. The ridges and the grooves may be definedby grooves longitudinally separated from each other with respect to thelength of the inner pipe 162. The plurality of grooves may intersecteach other or may be parallel to each other. The grooves may be straightgrooves extending parallel to the axis of the inner pipe 162 or may behelical grooves winding around the inner pipe 162.

In the double-wall pipe 160 of the sixth embodiment, the inner pipe 162is provided with annular grooves 162 g and multiple helical grooves 162d (e.g., three helical grooves). The edges of the ridges each formedbetween the adjacent helical grooves 162 d are close to the innersurface of the outer pipe 161. The diameter of a cylinder enveloping theridges of the inner pipe 162 is smaller than the inside diameter of theouter pipe 161. Thus, passages are defined by the helical grooves 162 dof the inner pipe 162 and the outer pipe 161, and by the ridges of theinner pipe 162 and the outer pipe 161. The ridges of the inner pipe 162are partially in contact with the outer pipe 161. Consequently, thepassage defined by the ridges of the inner pipe 162 and the outer pipe161 can be partially narrowed or partially blocked.

The annular grooves 162 g are provided to extend along thecircumferential direction of the inner pipe 162 at positionscorresponding to the inlet pipe 163 b and the outlet pipe 164 b,respectively. The annular grooves 162 g are provided to extend and windentirely around the inner pipe 162.

The helical grooves 162 d extend continuously between the two annulargrooves 162 g. For example, the helical grooves 162 d extend from one ofthe annular grooves 162 g to the other one of the annular grooves 162 g.Thus, the helical grooves 162 d form a longitudinal passage extend tothe annular grooves 162 g. The helical grooves 162 d extend continuouslybetween the opposite annular grooves 162 g.

Accordingly, the inlet pipe 163 b and the outlet pipe 164 b communicatedirectly with the annular grooves 162 g, respectively. In thisembodiment, the annular grooves 162 g and the helical grooves 162 d forma passage 160 a between the outer pipe 161 and the inner pipe 162.

The inlet pipe 163 b and the outlet pipe 164 b radially communicate withthe annular grooves 162 g of the inner pipe 162, respectively.Consequently, the high-pressure refrigerant is able to flow smoothlyinto and out of the passage 160 a.

Since the annular grooves 162 g are provided to correspond to the inletpipe 163 b and the outlet pipe 164 b, respectively, the circumferentialpositioning of the inner pipe 162 relative to the inlet pipe 163 b andthe outlet pipe 164 b attached to the outer pipe 162 is not necessary.Thus, the annular grooves 162 g and the helical grooves 162 b can beeasily connected to the inlet pipe 163 b and the outlet pipe 164 b.

The inside diameter of the outer pipe 162 is made slightly greater thanthe outside diameter of the inner pipe 162, the respective opposite endparts of the outer pipe 161 and the inner pipe 162 are joined together,and the outer pipe 161 including the cylindrical end parts 161 c has afixed inside diameter. Therefore, the outer pipe 161 and the inner pipe162 can be easily connected. Further, the passage 160 a can communicatewith the inlet pipe 163 b and the outlet pipe 164 b without partlyexpanding the outer pipe 161.

In the double-wall pipe 160 of the sixth embodiment, thehigh-temperature high-pressure refrigerant flows from a condenserthrough the passage 160 a to an evaporator, and the low-temperaturelow-pressure refrigerant flows from the evaporator through the innerpipe 162 to a compressor. The temperature of the high-temperaturehigh-pressure refrigerant is higher than that of the low-temperaturelow-pressure refrigerant and that of the atmosphere surrounding theouter pipe 161, and, the high-temperature high-pressure refrigerantneeds cooling in a refrigerant cycle device. Therefore, thehigh-temperature high-pressure refrigerant can be effectively cooled bythe atmosphere in addition to being cooled by heat transfer from thehigh-temperature high-pressure refrigerant to the low-temperaturelow-pressure refrigerant flowing through the inner pipe 162. Since thehigh-temperature high-pressure refrigerant flows through the widesubstantially annular passage 160 a defined by heat transfer surfaces ofa large area, heat is transferred efficiently from the high-temperaturehigh-pressure refrigerant to the low-temperature low-pressurerefrigerant. The helical grooves 162 d of the inner pipe 162 generate aturbulent stream in the passage 160 a, which promotes heat transfer.

The double-wall pipe 160 can be mounted to a vehicle. Bend portions canbe formed in the double-wall pipe 160 to locate the double-wall pipe 160at a suitable position of the vehicle. Since the helical grooves 162 dare extended in the substantially whole inner pipe 162 excluding the endparts, the passage 160 a maintains a necessary sectional area even whenthe double-wall pipe 160 is bent. For example, the helical grooves 162 dprevent the excessive deformation of the inner pipe 162. The helicalgrooves 162 d maintain the passage 160 a even when the outer pipe 161and the inner pipe 162 are deformed when the double-wall pipe 160 isbent. Since the inner pipe 162 provided with the helical grooves 162 dfunctions like a bellows, the inner pipe 162 can be easily bent.Therefore, it is preferable that the inner pipe 162 is provided with thehelical grooves 162 d at least in parts thereof to be bent.

The inner pipe 162 of the double-wall pipe 160 in the sixth embodimentmay be provided with straight grooves like the straight grooves 162 b ofthe inner pipe 162 of the double-wall pipe 160 in the first embodimentinstead of the helical grooves 162 d or may be provided with the helicalgrooves 162 d and the straight grooves 162 b in combination. The helicalgrooves 162 d may be partly broken with respect to the length of theinner pipe 162. The plurality of helical grooves 162 d may bediscontinuous. The inner pipe 162 may be provided with circumferentialgrooves having the shape of a broken ring instead of the annular grooves162 g. The annular grooves 162 g may be replaced with helical grooves ofvery small pitches having very narrow helical ridges. The annulargrooves 162 g may be omitted and the helical grooves 162 d and thestraight grooves 162 b may be extended between parts connected to thepipes 163 b, 164 b.

Other Embodiments

Although the present invention has been described in connection withsome preferred embodiments thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications willbecome apparent to those skilled in the art.

For example, the grooves (162 b, 162 d, 162 e, 162 f) of the foregoingembodiments may extend continuously over the entire length of the innerpipe 162. Alternatively, the grooves (162 b, 162 d, 162 e, 162 f) may belongitudinally divided into a plurality of separated sections. When thehelical grooves (162 d, 162 e, 162 f) are formed so as to intersect eachother, the helical grooves can be joined at the intersections of thehelical grooves (162 d, 162 e, 162 f), and the passage 160 a can besurely secured.

The grooves (162 b, 162 d, 162 e, 162 f) of the foregoing embodimentsare formed by deforming the wall of the inner pipe 162 so that thegrooves and the ridges are formed inside and outside the inner pipe 162.Grooves may be formed only in the outer surface of the inner pipe 162.The outer pipe 161 may be provided with grooves. For example, the outerpipe 161 may be provided with a plurality of intersecting helicalgrooves.

In the above-described first embodiment, as shown in FIG. 4, the outsidewall surface of the inner pipe 162 does not contact the inside wallsurface of the outer pipe 161. However, the outside wall surface of theinner pipe 162 can be made to partially contact the inside wall surfaceof the outer pipe 161, as shown in FIG. 12. Even in this case, becausethe groove 162 b is formed, a deformation of the inner pipe 162 due tothe outer pipe 161 can be restricted when the bend portions 160 c isformed. Furthermore, the passage 160 a can be easily formed in the bendportion 160 c by the groove 162 b.

Refrigerants (fluids) respectively having different physical propertiesmay flow through the double-wall pipe. Refrigerants flowing respectivelyin different directions, refrigerants respectively having differenttemperatures or refrigerants respectively having different pressures maybe used in the double-wall pipe. For example, a combination of ahigh-pressure refrigerant and a low-pressure refrigerant on the inletand the outlet side of the expansion valve, a combination of ahigh-pressure refrigerant and a low-pressure refrigerant on the suctionand the discharge sides of the compressor, or a combination of ahigh-temperature refrigerant on the inlet side of the condenser and alow-temperature refrigerant on the outlet side of the evaporator may beused. The double-wall pipe of the present invention can be used tosupply and return lines connecting the interior and the exterior unit ofa refrigerant cycle device. The double-wall pipe of the presentinvention can be applied to lines connecting the components of aninterior unit and those of an exterior unit of a refrigerant cycledevice.

The 6/8 in. pipe as the outer pipe 161 and the ⅝ in. pipe as the innerpipe 162 are only examples, and the outer pipe 161 and the inner pipe162 may be pipes of other sizes. For example, the inner pipe 162 may bea 6/8 in. pipe and the outer pipe 161 may be a 22 mm diameter pipehaving an inside diameter of 19.6 mm, the outer pipe 161 may be a ⅝ in.pipe and the inner pipe 162 may be a 12.7 mm diameter pipe having aninside diameter of 10.3 mm.

The double-wall pipe 160 does not need to be provided with the expandedparts 160 b and the grooves 162 b, 162 d, 162 e and 162 f and may beprovided with some of those.

Although the double-wall pipe 160 of the invention has been described asused for a refrigerant cycle device 100A of an automotive airconditioning system 100, the present invention is not limited thereto inits practical application. The double-wall pipe 160 may be applied todomestic air conditioners. When the double-wall pipe 160 is used for adomestic air conditioner, the temperature of the atmosphere around theouter pipe 161 is lower than that of air in the engine room 1.Therefore, the low-pressure refrigerant may be passed through thepassage 160 a and the high-pressure refrigerant may be passed throughthe inner pipe 162 when the mode of heat transfer from the high-pressurerefrigerant to the low-pressure refrigerant permits.

Although the double-wall pipes in the foregoing embodiments have beendescribed as heat exchangers for transferring heat from the refrigerantin one condition to the refrigerant in another condition, thedouble-wall pipes can be applied to heat exchange between differentfluids (e.g., water and a refrigerant). For example, water and therefrigerant may be passed through the inner pipe and the passage betweenthe outer and the inner pipes, respectively, or the refrigerator andwater may be passed through the passage and the inner pipe,respectively. A fluid to be passed through the passage between the outerpipe and the inner pipe can be selectively determined taking intoconsideration whether or not the fluid needs to exchange heat with theatmosphere and/or the flow rate of the fluid.

While the invention has been described with reference to preferredembodiments thereof, it is to be understood that the invention is notlimited to the preferred embodiments and constructions. The invention isintended to cover various modification and equivalent arrangements.Further, while the various elements of the embodiments are shown invarious combinations, which are preferred, other combinations andconfiguration, including more, less or only a single element, are alsowithin the spirit and scope of the invention.

1. A double-wall pipe comprising: an outer pipe provided with first andsecond openings, respectively, at first and second end parts of theouter pipe in a pipe longitudinal direction; an inner pipe inserted inthe outer pipe to define a passage between the outer pipe and the innerpipe; an inlet portion connected to the outer pipe to communicate withthe passage through the first opening; and an outlet portion connectedto the outer pipe to communicate with the passage through the secondopening, wherein: the outer pipe and the inner pipe are disposed todefine an expanded portion having an expanded sectional area in thepassage; and the expanded portion is provided at least at a portion nearthe inlet portion and the outer portion.
 2. The double-wall pipeaccording to claim 1, wherein the expanded portion is provided byexpanding at least a part of a circumferential portion of the outer pipein a circumferential direction, at least at a portion near the inletportion and the outlet portion.
 3. The double-wall pipe according toclaim 1, wherein the expanded portion is provided by reducing at least apart of a circumferential portion of the inner pipe in a circumferentialdirection, at least at a portion near the inlet portion and the outletportion.
 4. The double-wall pipe according to claim 1, wherein the innerpipe has therein a passage through which a fluid different from a fluidof the passage between the inner pipe and the outer pipe flows.
 5. Thedouble-wall pipe according to claim 1, wherein a surface of the innerpipe has a plurality of grooves.
 6. The double-wall pipe according toclaim 5, wherein the grooves are straight grooves extending in alongitudinal direction of the inner pipe.
 7. The double-wall pipeaccording to claim 5, wherein the grooves are helical grooves windingaround the inner pipe and extending in a longitudinal direction of theinner pipe.
 8. The double-wall pipe according to claim 5, wherein thegrooves include straight grooves extending in a longitudinal directionof the inner pipe, and helical grooves winding around the inner pipe andextending in the longitudinal direction of the inner pipe.
 9. Thedouble-wall pipe according to claim 7, wherein the helical groovesinclude first helical grooves winding in a first direction around theinner pipe, and second helical grooves winding in a second directionopposite the first direction around the inner pipe.
 10. A double-wallpipe comprising: an outer pipe; and an inner pipe inserted in the outerpipe to define a passage between the outer pipe and the inner pipe,wherein a surface of the inner pipe has a plurality of grooves.
 11. Thedouble-wall pipe according to claim 10, wherein the grooves are straightgrooves extending in a longitudinal direction of the inner pipe.
 12. Thedouble-wall pipe according to claim 10, wherein the grooves are helicalgrooves winding around the inner pipe and extending in a longitudinaldirection of the inner pipe.
 13. The double-wall pipe according to claim10, wherein the grooves include straight grooves extending in alongitudinal direction of the inner pipe, and helical grooves windingaround the inner pipe and extending in the longitudinal direction of theinner pipe.
 14. The double-wall pipe according to claim 12, wherein thehelical grooves include first helical grooves winding in a firstdirection around the inner pipe, and second helical grooves winding in asecond direction opposite the first direction around the inner pipe. 15.A double-wall pipe comprising: an inner pipe in which a fluid flows; andan outer pipe disposed at an outer side of the inner pipe to define apassage between the inner pipe and outer pipe, through which a fluidflows, wherein: the inner pipe is provided in its wall with a grooveportion extending from a first end part to a second end part of theinner pipe; the outer pipe has a first joining part joined airtightly tothe inner pipe at the first end part; and the outer pipe has a firstconnecting hole which is opened in a radial direction to directlycommunicate with the groove portion at the first end part.
 16. Thedouble-wall pipe according to claim 15, wherein the outer pipe has asecond joining part joined airtightly to the inner pipe at the secondend part; and the outer pipe has a second connecting hole which isopened in the radial direction to directly communicate with the grooveat the second end part.
 17. The double-wall pipe according to claim 15,wherein the groove portion has a groove extending in a circumferentialdirection at least in a part corresponding to the connecting hole of theouter pipe.
 18. The double-wall pipe according to claim 17, wherein thegroove extends in a complete circle in the circumferential direction atleast in the part corresponding to the connecting hole of the outerpipe.
 19. The double-wall pipe according to claim 15, wherein the grooveportion includes a helical groove extending helically.
 20. Thedouble-wall pipe according to claim 19, wherein the groove portionfurther includes a straight groove extending from the first end part tothe second end part.
 21. The double-wall pipe according to claim 15,wherein: the inner pipe has cylindrical end parts respectively formed inthe first and second end parts thereof; the outer pipe has cylindricalend parts formed at first and second end parts thereof and having aninside diameter slightly greater than an outside diameter of thecylindrical end parts of the inner pipe; and the cylindrical end partsof the outer pipe are directly airtightly joined to the cylindrical endparts of the inner pipe, respectively, to form joints.
 22. Thedouble-wall pipe according to claim 21, wherein the outer pipe includingthe cylindrical end parts has a fixed inside diameter.
 23. Thedouble-wall pipe according to claim 21, wherein: the outer pipeincluding the cylindrical end parts has a fixed inside diameter; andparts, forming the joints, of the cylindrical end parts of the outerpipe are radially reduced so as to tightly contact the inner pipe. 24.The double-wall pipe according to claim 10, wherein the outer pipe andthe inner pipe have a bend portion which is bent in a predeterminedshape in accordance with a mounting space.
 25. The double-wall pipeaccording to claim 1, wherein the double-wall pipe is used in arefrigerant cycle device including a compressor, a condenser, apressure-reducing device, an evaporator, wherein: the passage betweenthe outer pipe and the inner pipe, and a passage inside the inner pipeare used, respectively, as at least a part of a high-pressure passageconnecting the condenser and the pressure-reducing device to carry ahigh-pressure refrigerant, and as at least a part of a low-pressurepassage connecting the evaporator and the compressor to carry alow-pressure refrigerant.
 26. The double-wall pipe according to claim10, wherein the outer pipe and the inner pipe are used for a refrigerantcycle device such that a high-pressure refrigerant flows through thepassage between the outer pipe and the inner pipe, and a low-pressurerefrigerant flows in the inner pipe.
 27. The double-wall pipe accordingto claim 1, wherein the outer pipe and the inner pipe are formedseparately.
 28. A double-wall pipe for a refrigerant cycle device,comprising: an inner pipe for carrying a low-pressure refrigerant afterdecompressed in the refrigerant cycle device flows, the inner pipehaving an uneven portion provided in its wall; and an outer pipedisposed at an outer side of the inner pipe to define a passage betweenthe inner pipe and the outer pipe, through which a high-pressurerefrigerant before being decompressed in the refrigerant cycle deviceflows.
 29. The double-wall pipe according to claim 28, wherein theuneven portion includes at least a groove extending in a longitudinaldirection of the inner pipe.
 30. The double-wall pipe according to claim29, wherein the groove is a helical groove.
 31. The double-wall pipeaccording to claim 28, wherein: the uneven portion includes ridges andgrooves relative to an outer surface of the inner pipe; edges of theridges of the uneven portion of the inner pipe are rounded in a radiussmaller than an inner radius of the outer pipe; and the passage for thehigh-pressure refrigerant is defined by the outer pipe and the groovesof the inner pipe and by the outer pipe and the ridges of the innerpipe.
 32. The double-wall pipe according to claim 28, wherein: the innerpipe has an inner cylindrical end part without having the unevenportion, at one end part of the inner pipe; the outer pipe has an outercylindrical end part at a part corresponding to the inner cylindricalend part of the inner pipe and having an inside diameter slightlygreater than an outside diameter of the inner cylindrical end part ofthe inner pipe; and the outer cylindrical end part of the outer pipe andthe inner cylindrical end part of the inner pipe are directly airtightlyjoined to form a joint.
 33. The double-wall pipe according to claim 1,wherein the double-wall pipe is used for an air conditioner for avehicle.
 34. A refrigerant cycle device comprising: a compressor whichcompresses refrigerant; a refrigerant radiator which cools high-pressurerefrigerant from the compressor; a pressure-reducing unit whichdecompresses the high-pressure refrigerant to be a low-pressurerefrigerant; an evaporator in which the low-pressure refrigerant afterdecompressed in the pressure-reducing unit is evaporated; and adouble-wall pipe including an outer pipe and an inner pipe, wherein theinner pipe is inserted into the outer pipe to define a first passagebetween the outer pipe and the inner pipe, and has therein a secondpassage, wherein: the first passage is used as at least a part of ahigh-pressure passage through which the high-pressure refrigerant fromthe refrigerant radiator to the pressure-reducing unit flows; the secondpassage is used at least a part of a low-pressure passage through whichthe low-pressure refrigerant from the evaporator to the compressorflows; the outer pipe has first and second openings, respectively, incircumferential wall portions of first and second end parts of the outerpipe in a pipe longitudinal direction; the outer pipe is connected to aninlet portion from which the high-pressure refrigerant flows into thefirst passage, and is connected to an outlet portion from which thehigh-pressure refrigerant in the first passage flows out; the outer pipeand the inner pipe are disposed to define an expanded portion having anexpanded sectional area in the first passage; and the expended portionis provided at least at a portion near the inlet portion and the outerportion.
 35. The refrigerant cycle device according to claim 34, whereinthe expanded portion is provided by expanding at least a part of acircumferential portion of the outer pipe in a circumferentialdirection, at least in a portion near the inlet portion and the outletportion.
 36. The refrigerant cycle device according to claim 34, whereinthe expanded portion is provided by reducing at least a part of acircumferential portion of the inner pipe in a circumferentialdirection, at least in a portion near the inlet portion and the outletportion.
 37. A refrigerant cycle device comprising: a compressor whichcompresses refrigerant; a refrigerant radiator which cools high-pressurerefrigerant from the compressor; a pressure-reducing unit whichdecompresses the high-pressure refrigerant to be a low-pressurerefrigerant; an evaporator in which the low-pressure refrigerant afterdecompressed in the pressure-reducing unit is evaporated; and adouble-wall pipe including an outer pipe and an inner pipe, wherein theinner pipe is inserted into the outer pipe to define a first passagebetween the outer pipe and the inner pipe, and has therein a secondpassage, wherein: the first passage is used as at least a part of ahigh-pressure passage through which the high-pressure refrigerant fromthe refrigerant radiator to the pressure-reducing unit flows; the secondpassage is used at least a part of a low-pressure passage through whichthe low-pressure refrigerant from the evaporator to the compressorflows; and a surface of the inner pipe has a plurality of grooves. 38.The refrigerant cycle device according to claim 37, wherein thedouble-wall pipe is bent in accordance with a mounting space to have abend portion.
 39. The refrigerant cycle device according to claim 37,wherein the grooves are straight grooves extending in a longitudinaldirection of the inner pipe.
 40. The refrigerant cycle device accordingto claim 37, wherein the grooves are helical grooves winding around theinner pipe and extending in a longitudinal direction of the inner pipe.41. The refrigerant cycle device according to claim 37, wherein thegrooves include straight grooves extending in a longitudinal directionof the inner pipe, and helical grooves winding around the inner pipe andextending in the longitudinal direction of the inner pipe.
 42. Therefrigerant cycle device according to claim 34, wherein the low-pressurerefrigerant at a refrigerant outlet of the evaporator has a superheatdegree that is lower than a predetermined value.